The invention relates to a device for detecting a centring error of a rotationally symmetrical surface of an object, which device comprisies a radiation source for exposing a small portion of the surface, a rotatable holder for the object and a radiation-sensitive detection system arranged in the path of a beam originating from the surface and supplying a signal which is a measure of the centring error with respect to the rotational axis of the holder.
A device of this type is used, for example, for centring a lens element in its holder before this element is fixed in the holder. Particularly when composing a lens system comprising a large number of lens elements such as a projection lens system, it is important to align each of the lens elements accurately, that is to say, to ensure that the optical axis of each lens element accurately coincides with a reference axis, which is generally the axis of the holder in which all lens elements must be arranged. In the assembly of such a lens system a first lens element is centred with respect to a reference axis and is fixed, subsequently a second lens element is centred with respect to the same reference axis and is fixed, and so forth until all lens elements are fixed in the holder. Each of the refractive surfaces of each lens element must be centred separately. In the case of a spherical surface this means that it must be ascertained whether the centre of curvature of the relevant surface is located on the reference axis. The different rotationally symmetrical sectors of an aspherical rotationally symmetrical surface have different centres of curvature which are located on one line. Centring of such a surface means that coincidence of this line with the reference axis is to be ensured.
In order to detect a centring error the lens element may be rotated around an axis which coincides with the reference axis and a radiation beam can be directed onto the surface to be centred. If the lens surface is not correctly centred with respect to the rotational axis, this surface will make an oscillatory movement. This movement can be detected by a radiation-sensitive detection system arranged in the path of a beam reflected by the surface, which system in principle supplies one output signal, but suitably more and preferably four output signals. By processing these signals in the correct manner the movement of the radiation spot formed on the detection system and hence the movement of the beam and consequently also the centring error of the lens surface can be detected.
A radiation-sensitive detection system of this type may be in the form of, for example, a so-called four-quadrant detector consisting of four separate detectors which are arranged in the four quadrants of an imaginary X-Y system of coordinates. When using such a detector, however, the radiation spot must be pre-aligned with respect to the detector in order that the radiation spot is not imaged on only one or two of the detectors in the case of correct centring of the lens surface.
The radiation-sensitive detection system may also be in the form of a so-called two-dimensional position-sensitive photo diode. Such a photo diode comprises a single semiconductor element having two electrodes or, in the case of a two-dimensional photo diode, four electrodes in which the difference between the output signals at two facing electrodes is a measure of the position of the centre of the intensity profile of a radiation spot formed on this photo diode along the axis connecting the two electrodes. The signals which are supplied by such a position-sensitive photodiode are, however, not only dependent on the position of the radiation spot on this photo diode but also on the intensity variation in the radiation beam.
When used in the device described, both the four-quadrant detector and the position-sensitive photodiode supply signals having a small signal-to-noise ratio due to the low intensity of the radiation spot on these detectors. This low intensity is the result of the low reflection coefficient of the surface to be centred. The said device using a four-quadrant detector of a position-sensitive photodiode is less suitable in practice for high-precision centring of a lens surface. Moreover, the centring error signal in the device described may be affected by irregularities in the lens surface, for example, by errors in the coating of this surface.